The field of the invention is high power x-ray tubes used in computer aided tomography, angiography, and cineradiography, and more specifically, the cooling of the target structure in such tubes.
An x-ray tube includes a glass or metal envelope which encloses in a near vacuum a cathode electrode and a target structure which forms an anode electrode. The cathode is heated to produce electrons and a high voltage is applied across the electrodes to propel the electrons at the target material. When the electrons strike the target, x-rays and heat are produced. The x-rays are directed through a window in the envelope to perform their useful function, while the heat is dissipated through the walls of the envelope.
As the power of the x-ray tube increases, the measures required to effectively dissipate the heat become more demanding. The target is constructed of a material, such as tungsten, which can be operated at high temperatures and its mounting structure is typically coated with a high emissivity material which radiates heat to the surrounding envelope. In some industrial application where metal envelopes are used, the interior surface of the envelope may also be coated with a high emissivity material which absorbs the radiated heat from the target. Heat radiating fins or a manifold for conveying a cooling liquid may be formed on the outer surface of the envelope to remove the heat.
While such radiant and convective heat transfer strategies cool the target structure, they do not keep the temperature of the target material directly in the path of the electron beam sufficiently cool when high powered x-ray pulses are required. As described in U.S. Pat. Nos. 3,869,634; 4,187,442; 4,272,696; 4,393,511; and 4,569,070, the recognized solution to this problem is to form the target on a disc, and to rotate the disc such that the target material which is subjected to the electron bombardment is continuously changed. For example, the tungsten target material may be deposited as a band, or focal track, around the periphery of the disc, and the disc is rotated at a speed of from 3,000 to 10,000 revolutions per minute. Although such rotation reduces localized heating of the target material, it complicates the cooling of the target structure since the target structure now includes a rapidly rotating disc driven by a motor. In addition, a vastly increased amount of heat is produced because of the increased power levels which can be achieved.